whats the best/quickest way of accessing an already the windows current fragment's depth in a shader.
im wanting to fog at the end of the renderloop thus i need to access the current buffers depth values.

will the ability to read the current fragments depth value ever exist in glsl

what im trying to do is semi-standard (non volumteric) fog, what is a nice way of doing it with shaders?
A/ create a depthtexture of the scene
B/ draw a polygon over the scene using the depth texture to govern how much to fog
are there any better methods? as in looks etc